In a typical cellular radio system, radio or wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (in a Universal Mobile Telecommunications System (UMTS) network) or “eNodeB” (in a Long Term Evolution (LTE) network). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UEs) within range of the base stations.
In some radio access networks, several base stations may be connected (e.g., by landlines or microwave) to a radio network controller (RNC) or a base station controller (BSC). The radio network controller supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using a wideband code division multiple access (WCDMA) air interface between user equipment units (UEs) and the radio access network (RAN).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. The first release for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification has issued, and as with most specifications, the standard is likely to evolve. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology where the radio base station nodes are connected to a core network (via Access Gateways (AGWs)) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeBs in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has what is sometimes termed a “flat” architecture including radio base station nodes without reporting to radio network controller (RNC) nodes.
Transmission and reception from a node, e.g., a radio terminal like a UE in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). In Frequency Division Duplex (FDD), as illustrated to the left in FIG. 1, downlink and uplink transmission take place in different, sufficiently separated, frequency bands. In Time Division Duplex (TDD), as illustrated to the right in FIG. 1, downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired frequency spectrum, whereas FDD requires paired frequency spectrum.
Typically, a transmitted signal in a communication system is organized in some form of frame structure. For example, LTE uses ten equally-sized subframes 0-9 of length 1 ms per radio frame as illustrated in FIG. 2.
In the case of FDD operation (illustrated in the upper part of FIG. 2), there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission (fDL). At least with respect to the radio terminal in a cellular communication system, FDD can be either full duplex or half duplex. In the full duplex case, a terminal can transmit and receive simultaneously, while in half-duplex operation (see FIG. 1), the terminal cannot transmit and receive simultaneously (although the base station is capable of simultaneous reception/transmission, i.e., receiving from one terminal while simultaneously transmitting to another terminal). In LTE, a half-duplex radio terminal monitors/receives in the downlink except when explicitly instructed to transmit in the uplink in a certain subframe.
In the case of TDD operation (illustrated in the lower part of FIG. 2), there is only a single carrier frequency, and uplink and downlink transmissions are separated in time also on a cell basis. Because the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither downlink nor uplink transmissions occur in order to avoid interference between uplink and downlink transmissions. For LTE, special subframes (subframe 1 and, in some cases, subframe 6) provide this guard time. A TDD special subframe is split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to uplink or downlink transmission.
Time division duplex (TDD) allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. In LTE, there are seven different configurations as shown in FIG. 3. The configurations cover a wide range of allocations from uplink heavy with a DL to UL ratio of 2:3 (Configuration 0) to downlink heavy with a DL to UL ratio of 9:1 (Configuration 5).
To avoid significant interference between downlink and uplink transmissions between different cells, neighbour cells should have the same downlink/uplink configuration. Otherwise, uplink transmission in one cell may interfere with downlink transmission in the neighbouring cell (and vice versa) as illustrated in FIG. 4 where the uplink transmission of the UE in the right cell (MS2) is interfering with the downlink reception by the UE in the left cell (MS1). As a result, the downlink/uplink asymmetry does not vary between cells. The downlink/uplink asymmetry configuration is signalled as part of the system information and remains fixed for a long period of time.
Heterogeneous networks refer to cellular networks deployed with base stations having different characteristics, mainly in terms of output power, and overlapping in coverage.
The term hierarchical cell structures is used to refer to one type of heterogeneous network deployment. One simple example of a heterogeneous network is a macro cell overlaying one or more low power nodes (LPNs) such as pico cells or femto cells (also known as home eNBs).
A characteristic of heterogeneous networks is that the output powers of different cells (at least partially) covering the same area are different. For example, the output power of a pico base station or a relay might be on the order of 30 dBm or less, while a macro base station might have a much larger output power of 46 dBm. Consequently, even in the proximity of the pico cell, the downlink signal strength from the macro cell can be larger than that of the pico cell.
Cell selection is typically based on received signal strength, i.e., the UE terminal connects to the strongest downlink. However, due to the difference in downlink transmission power between different cells, (e.g., macro and pico), this does not necessarily correspond to the best uplink. From an uplink perspective, it would be better to select a cell based on the uplink path loss as illustrated in FIG. 5 (the inverse of the uplink path loss is illustrated in dashed lines while the solid lines show the received downlink power from both cells/base stations). If uplink path loss is used as the cell selection criterion, the UE transmits uplink using a lower uplink transmit power than if downlink received power is used. This would be beneficial from a capacity perspective since it allows reuse of the radio resources used by one pico cell-connected UE in another pico cell (assuming a sufficient distance between both of these pico cells) because the one pico cell-connected UE's uplink transmission power (and hence interference) can be reduced compared to what it would be if that UE were connected to the macro cell. However, connecting to the best uplink cell is possible, even if the cell selection is based on downlink signal strength measurements, by assigning different measurement offsets to the different cells.
But connecting to the cell with the best uplink does not mean that the best downlink is necessarily used. This condition is sometimes referred to as link imbalance. If the two cells in FIG. 5 transmit on the same frequency, downlink transmissions from the pico cell are subject to strong interference from macro cell downlink transmissions, and in certain regions surrounding the pico base station, it may not be possible for a UE to receive the transmissions from the pico cell. In other words, macro-to-pico downlink interference prevents the UE from receiving from the pico cell.
Solving the uplink-downlink imbalance is important in heterogeneous networks. A simple solution is to operate different overlapping cells or cell “layers” on different (sufficiently separated) frequencies. One approach in situations where different frequencies cannot be used for different cell layers is to employ uplink desensitization by decreasing the receiver sensitivity in the pico base station such that the uplink and downlink cell boundaries coincide, i.e., the ‘Gray region’ in FIG. 5 surrounding the pico base station shrinks and eventually disappears. In LTE, decreasing the sensitivity is not required because a higher received power can be achieved by proper setting of the power control parameters, i.e., P0. This resolves the problem of receiving downlink transmissions from the pico cell at the cost of using a higher received power target in the pico cell.
As indicated above, time division duplex (TDD) networks typically use a fixed frame configuration where some subframes are uplink and some are downlink. This prevents or at least limits the flexibility to adopt the uplink/downlink resource asymmetry to varying traffic situations. Heterogeneous deployments typically separate the cell layers in frequency, which comes at a cost in terms of the spectrum required or the use of desensitization to mitigate the link imbalance problem, which artificially decreases uplink performance.
WO 2011/077288 describes an approach to mitigate these problems. In particular, WO 2011/077288 provides the ability for a subframe to be configured as a “flexible” subframe, which means that at least three different types of subframes can be configured in a TDD system: a downlink (DL) subframe, an uplink (UL) subframe and a “flexible” subframe. Each flexible subframe can be dynamically allocated to be an uplink subframe in one instance of a frame and a downlink subframe in another frame instance. Information is generated for a radio terminal indicating how the radio terminal should interpret or use one or more flexible subframes.
Downlink subframes (which exist in LTE Rel-8) are used (among other things) for transmission of downlink data, system information, control signalling and hybrid-ARQ (hybrid-automatic repeat request) feedback in response to uplink transmission activity. The UE is monitoring the physical downlink control channel (PDCCH) as in LTE Rel-8, i.e. it may receive scheduling assignments and scheduling grants. Special subframes (as shown in FIG. 2) are similar to downlink subframes except, in addition to the downlink part, they include also a guard period as well as a small uplink part in the end of the subframe to be used for random access or sounding.
Uplink subframes (which exist in LTE Rel-8) are used (among other things) for transmission of uplink data, uplink control signalling (channel-status reports), and hybrid-ARQ feedback in response to downlink data transmission activity. Data transmission on the physical uplink shared channel (PUSCH) in uplink subframes are controlled by uplink scheduling grants received on a PDCCH in an earlier subframe.
Flexible subframes as described in WO 2011/077288 (which are not specified in LTE Rel-8) can be used for uplink or downlink transmissions as determined by scheduling assignments/grants.